1. Field of the Invention
The present invention relates to a light-emitting element which includes a light-emitting layer containing an organic compound between a pair of electrodes, and a light-emitting device and a lighting device each including the light-emitting element.
2. Description of the Related Art
In recent years, a light-emitting element (also referred to as electroluminescent (EL) element) which includes a light-emitting layer (hereinafter also referred to as EL layer) containing an organic compound between a pair of electrodes has been actively developed. Lighting has attracted attention as one of the application fields of the light-emitting element. This is because a lighting device including the EL element has features different from those of other lighting devices; for example, the lighting device including the EL element can be thin and lightweight and can emit light from a surface having two dimensions.
In addition, an EL element has attracted attention also in terms of its high efficiency of conversion from electrical power into light and its high potential for saving energy. It is also a unique feature of an EL element that, depending on a substrate selected, it is possible to provide a lighting device having flexibility, a lighting device having high resistance against impact such as physical destruction, or a very lightweight lighting device.
Japanese Industrial Standards (ES) defines the color of lighting on the basis of correlated color temperature. Light with color temperatures ranging from 2600 K to 7100 K, from incandescent color with the lowest color temperature to daylight color with the highest color temperature, is defined as light for room lighting. Light of a variety of colors can be emitted from an EL element by selection of a light-emitting substance. However, light represented by such a color temperature generally has a wide wavelength, and it is very difficult to obtain light which satisfies the standard and the luminance and reliability required for lighting at the same time by using one light-emitting substance. In view of the above, an EL element in which a plurality of light-emitting substances with different emission wavelengths are used so that favorable white light emission can be obtained has been studied (e.g., see Patent Document 1).
[Reference]
    [Patent Document 1] Japanese Published Patent Application No. 2007-53090